Flow completion apparatus

ABSTRACT

A flow completion apparatus comprises a wellhead housing installed at an upper end of a well bore and a tubing spool connected over the wellhead housing. The tubing spool includes a central bore extending axially therethrough, a production outlet communicating with the central bore and an annulus passageway communicating with the tubing annulus. A tubing hanger is supported in the central bore and includes a production bore extending axially therethrough and a production passageway communicating between the production bore and the production outlet. The tubing spool comprises a workover passageway extending between the annulus passageway and a portion of the central bore located below the top of the tubing hanger. Furthermore, the tubing hanger comprises an annulus bore extending between the workover passageway and the top of the tubing hanger.

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/815,430 filed on Mar. 22, 2001, which is basedon U.S. Provisional Patent Application No. 60/192,124 filed on Mar. 24,2000 and U.S. Provisional Patent Application No. 60/268,329 filed onFeb. 12, 2001.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a flow completion apparatus forproducing oil or gas from a subsea well. More particularly, theinvention relates to a flow completion apparatus which comprises atubing hanger having an annulus bore which is adapted to communicatewith an annulus port in a tubing hanger running tool to provide forfluid communication between the tubing annulus and the choke and killline of a blowout preventer which is installed over the tubing hangerduring installation and workover of the flow completion apparatus.

[0003] Flow completion assemblies for producing oil or gas from subseawells are generally categorized as either conventional or horizontal. Atypical horizontal flow completion assembly, such as that disclosed inU.S. Pat. No. 6,039,119, comprises a wellhead housing which is installedat the upper end of the well bore, a tubing spool which is connected tothe top of the wellhead housing and which includes a central boreextending axially therethrough, an annular tubing hanger which issuspended in the central bore, and a tree cap which is installed in thecentral bore above the tubing hanger. The tubing hanger supports atleast one tubing string that extends into the well bore and defines atubing annulus surrounding the tubing string. In addition, the tubinghanger comprises a concentric production bore which communicates withthe tubing string and a lateral production passageway which extendsbetween the production bore and a production outlet in the tubing spool.The tubing spool also includes an annulus passageway which extends fromthe tubing annulus to an annulus outlet, and a workover passageway whichextends from the annulus passageway to a portion of the central borethat is located above the tubing hanger. These passageways provide forcommunication between the tubing annulus and the central bore above thetubing hanger during installation and workover of the flow completionassembly.

[0004] The regulations of certain countries pertaining to the subseaproduction of oil and gas require that the flow completion assemblyprovide at least two pressure-containing barriers between the well boreand the environment. In the typical horizontal flow completion assembly,the first barrier is provided by a wireline plug that is installed inthe production bore above the production passageway, in conjunction withan annular, usually metal seal which is positioned between the tubinghanger and the tubing spool above the production outlet. The secondbarrier is provided by the tree cap, which is sealed to the tubing spoolby an annular, typically metal seal and often includes an axial throughbore which in turn is sealed by a wireline plug or other suitableclosure member.

[0005] During installation of the flow completion assembly, the tubingspool is landed onto the wellhead housing, after which a blowoutpreventer (“BOP”) is installed onto the tubing spool by means of a riserdeployed from a surface vessel. The tubing hanger is then lowered on atubing hanger running tool (“THRT”) through the riser and the BOP andlanded in the central bore of the tubing spool. The THRT is thenretrieved and the tree cap is lowered on a dedicated tool, such as aTHRT, through the riser and the BOP and landed in the central boredirectly above the tubing hanger. After the tree cap is installed, theTHRT is retrieved, the BOP is retrieved, and the flow completionassembly is ready to be put into production.

[0006] During a typical workover of the flow completion assembly, theBOP and the riser are once again connected to the tubing spool, the treecap is usually removed from the tubing spool, and the THRT is connectedto the tubing hanger. Once the workover operations are completed, theTHRT is retrieved and the tree cap is re-installed through the riser andthe BOP. Then the THRT is retrieved, the BOP is retrieved, and the flowcompletion assembly is ready to be put back into production.

[0007] Since the tree cap is required to maintain well pressure in theevent of a failure of the first barrier, the tree cap typicallycomprises a rigid metal body and a robust metal lockdown mechanism tofirmly lock the body to the tubing spool. Consequently, the tree cap isusually too heavy to be installed by a remotely operated vehicle (“ROV”)and must instead be lowered from the surface vessel on a speciallydesigned tree cap running tool. Thus, installation of the tree caprequires a special running trip, both during installation of the flowcompletion assembly and after a workover operation. Each such triptypically requires a significant amount of valuable rig time tocomplete, which necessarily increases the cost of completing andmaintaining the well.

[0008] In addition, during retrieval of the THRT prior to installationof the tree cap, debris within the riser often falls into the centralbore of the tubing spool above the tubing hanger. Left unattended, thisdebris could foul the sealing surfaces of the central bore and therebyprevent the tree cap from forming an effective seal with the tubingspool. Therefore, before the tree cap is installed the central bore mustbe thoroughly cleaned, a process that consumes additional valuable rigtime and increases the cost of completing and maintaining the well.

[0009] Furthermore, during both installation and workover of the typicalhorizontal flow completion assembly, communication between the tubingannulus and the surface vessel is usually established through theannulus passageway, the workover passageway, and either the choke andkill lines of the BOP or the annulus between the THRT and the riser. Forexample, deep well circulation is often accomplished by pumping fluiddown the THRT, through the production bore, through the tubing string,up the tubing annulus, through the annulus passageway, through theworkover passageway, into the central bore above the tubing hanger andthrough the BOP choke and kill lines. One problem with this arrangementis that the flow in the central bore of the tubing spool above thetubing hanger is generally unrestricted, and this unrestricted flow canfoul the tubing hanger lockdown mechanism and erode the central bore,including the locking profile and the annular sealing surfaces withinthe central bore against which the tree cap must lock and seal,respectively.

SUMMARY OF THE INVENTION

[0010] In accordance with the present invention, these and otherdisadvantages in the prior art are overcome by providing a flowcompletion apparatus for controlling the flow of fluid through a tubingstring which extends into a well bore and defines a tubing annulussurrounding the tubing string. The flow completion apparatus comprises awellhead housing which is installed at an upper end of the well bore; atubing spool which is connected over the wellhead housing and whichincludes a central bore that extends axially therethrough, a productionoutlet which communicates with the central bore, and an annuluspassageway which communicates with the tubing annulus; a tubing hangerwhich is supported in the central bore and is connected to an upper endof the tubing string, the tubing hanger including a production borewhich extends axially therethrough and a production passageway whichcommunicates between the production bore and the production outlet; afirst closure member which is positioned in the production bore abovethe production passageway; and a first annular seal which is positionedbetween the tubing hanger and the central bore above the productionpassageway. Furthermore, the tubing spool also comprises a workoverpassageway which extends between the annulus passageway and a portion ofthe central bore that is located above the first seal, and the tubinghanger also comprises an annulus bore which extends between the workoverpassageway and the top of the tubing hanger. In this manner fluidcommunication between the tubing annulus and the top of the tubinghanger may be established through the annulus passageway, the workoverpassageway and the annulus bore.

[0011] In accordance with another aspect of the present invention, theflow completion apparatus further comprises a BOP which is removablyconnectable to the top of the tubing spool and which includes a BOPbore, a first set of BOP rams, and at least one choke and kill line thatcommunicates with a portion of the BOP bore which is located below thefirst BOP rams; and a THRT which is removably connectable to the top ofthe tubing hanger and which includes a generally cylindrical outerdiameter surface, a production port that communicates with theproduction bore, and an annulus port that communicates between theannulus bore and an opening which is formed in the outer diametersurface. Furthermore, the first BOP rams are adapted to sealingly engagethe outer diameter surface above the opening. In this manner fluidcommunication between the tubing annulus and the BOP choke and kill linemay be established through the annulus passageway, the workoverpassageway, the annulus bore, the annulus port and the portion of theBOP bore which is located below the first BOP rams.

[0012] In accordance with yet another aspect of the present invention,the BOP comprises a second set of BOP rams, the choke and kill linecommunicates with a portion of the BOP bore which is located between thefirst and second BOP rams, and the second BOP rams are adapted tosealingly engage the outer diameter surface below the opening. In thismanner, fluid communication between the tubing annulus and the BOP chokeand kill line may be established through the annulus passageway, theworkover passageway, the annulus bore, the annulus port and the portionof the BOP bore which is located between the first and second BOP rams.

[0013] It may therefore be seen that the annulus bore in the tubinghanger provides a convenient means for connecting the tubing annuluswith an annulus port in a THRT. In addition, the annulus port provides aclosed path between the annulus bore and a BOP choke and kill line.Thus, the flow of fluid between the tubing annulus and the BOP choke andkill line will be restricted by the annulus bore and the THRT.Consequently, this flow will not erode or contaminate the central boreof the tubing spool.

[0014] In accordance with yet another aspect of the present invention, afirst barrier between the well bore and the environment is provided byboth the first closure member in the production bore and the firstannular seal between the tubing hanger and the tubing spool. Inaddition, a second barrier between the well bore and the environment isprovided by both a second closure member that is positioned in theproduction bore above the first closure member, and a second annularseal that is positioned between the tubing hanger and the tubing spoolabove the first seal. In this manner, both the first and the secondbarriers between the well bore and the environment are mounted in or onthe tubing hanger. Consequently, the flow completion apparatus does notrequire a tree cap which is capable of sealing against the pressure inthe well bore. Instead, a simple, lightweight debris cap can beinstalled on the tubing spool using an ROV, thereby saving the time andcost required to run a tree cap from a surface vessel duringinstallation and workover operations.

[0015] These and other objects and advantages of the present inventionwill be made apparent from the following detailed description, withreference to the accompanying drawings. In the drawings, the samereference numbers are used to denote similar components in the variousembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a representation of a prior art horizontal flowcompletion assembly shown in the production mode of operation;

[0017]FIG. 2 is a representation of a prior art horizontal flowcompletion assembly shown in the installation or workover mode ofoperation;

[0018]FIG. 3 is a representation of the flow completion apparatus of thepresent invention shown in the production mode of operation;

[0019]FIG. 4 is a representation of the flow completion apparatus ofFIG. 3 shown in the installation or workover mode of operation with afirst embodiment of a BOP connected to the tubing spool;

[0020]FIG. 5 is a representation of the flow completion apparatus ofFIG. 3 shown in the installation or workover mode of operation with asecond embodiment of a BOP connected to the tubing spool;

[0021]FIG. 6 is a representation of the flow completion apparatus ofFIG. 3 shown with a light weight intervention package connected to thetubing hanger;

[0022]FIG. 7 is a representation of a second embodiment of the flowcompletion apparatus of the present invention shown in the installationor workover mode of operation;

[0023]FIG. 8 is a representation of a third embodiment of the flowcompletion apparatus of the present invention shown in the productionmode of operation;

[0024]FIG. 9 is a representation of the flow completion apparatus ofFIG. 8 shown in the installation or workover mode of operation;

[0025]FIG. 10 is a representation of a fourth embodiment of the flowcompletion apparatus of the present invention shown in the productionmode of operation;

[0026]FIG. 11 is a representation of a fifth embodiment of the flowcompletion apparatus of the present invention shown in the installationor workover mode of operation;

[0027]FIG. 12 is a representation of a sixth embodiment of the flowcompletion apparatus of the present invention shown in the installationor workover mode of operation;

[0028]FIG. 13 is a bottom view of the THRT shown in FIG. 12;

[0029]FIG. 14 is a longitudinal cross-sectional view of yet anotherembodiment of the flow completion apparatus of the present inventionshown in the installation mode;

[0030]FIGS. 15 through 17 are enlarged views of portions of the flowcompletion apparatus shown in FIG. 14;

[0031]FIG. 18 is a longitudinal cross-sectional view of still anotherembodiment of the flow completion apparatus of the present inventionshown in the production mode;

[0032]FIG. 19 is a top view of the tree cap component of the flowcompletion apparatus shown in FIG. 18;

[0033]FIG. 19A is a cross-sectional view of the tree cap taken alongline A-A of FIG. 19;

[0034]FIG. 19B is a cross-sectional view of the tree cap taken alongline B-B of FIG. 19;

[0035]FIG. 19C is a cross-sectional view of the tree cap taken alongline C-C of FIG. 19;

[0036]FIG. 20 is a top view of the locking cap component of the flowcompletion apparatus shown in FIG. 18;

[0037]FIG. 21 is a representation of a further embodiment of the flowcompletion apparatus of the present invention shown in the installationor workover mode of operation; and

[0038]FIG. 22 is a representation of yet another embodiment of the flowcompletion apparatus of the present invention shown in the installationor workover mode of operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] Referring to FIG. 1, a prior art horizontal-type flow completionassembly 10 is shown to comprise a wellhead housing 12 which isinstalled at the upper end of a well bore (not shown), a tubing spool 14which is connected and sealed to the top of the wellhead housing andwhich comprises a central bore 16 extending axially therethrough, agenerally annular tubing hanger 18 which is suspended from a shoulder(not shown) located in the central bore, and a tree cap 20 which isinstalled in the central bore above the tubing hanger. The tubing hanger18 is secured to the tubing spool 14 by a lockdown mechanism (not shown)and supports at least one tubing string 22 which extends into the wellbore and defines a tubing annulus 24 surrounding the tubing sting. Inaddition, the tubing hanger 18 includes a production bore 26 whichcommunicates with the tubing string 22 and a lateral productionpassageway 28 which extends between the production bore and the outerdiameter of the tubing hanger. The tubing spool 14 includes a productionoutlet 30 which communicates with the production passageway 28, anannulus passageway 32 which communicates with the tubing annulus 24, anannulus outlet 34 which is connected to the annulus passageway, and aworkover passageway 36 which extends between the annulus passageway andthe portion of the central bore 16 above the tubing hanger 18. Thetubing hanger 18 is sealed to the tubing spool 14 by a lower, typicallymetal seal ring 38 and an upper, also typically metal seal ring 40. Inaddition, the production bore 26 is sealed above the productionpassageway 28 by means of a wireline plug 42, which directs the flow ofoil or gas from the tubing string 22 into the production outlet 30. Aproduction master valve 44 and a production wing valve 46 are providedto control flow through the production outlet 30, while an annulusmaster valve 48, an annulus wing valve 50 and an workover valve 52 areprovided to control flow through the annulus passageway 32, the annulusoutlet 34 and the workover passageway 36, respectively.

[0040] During the production mode of operation of the flow completionassembly 10, which is depicted in FIG. 1, a first barrier between thewell bore and the environment is provided by the upper seal ring 40 andthe wireline plug 42. The second barrier is provided by the tree cap 20,more particularly, by a typically metal seal ring 54 which is disposedbetween the tree cap and the tubing spool 14 and a wireline plug 56which is positioned in an axial bore 58 extending through the tree cap.Thus, in the prior art flow completion assembly 10, the first barrier isassociated with the tubing hanger 18 while the second barrier isassociated with the tree cap 20. Although not shown in FIG. 1, the treecap 20 also includes a lockdown mechanism to secure the tree cap to thetubing spool 14.

[0041] Referring to FIG. 2, the flow completion assembly 10 is shown inthe installation or workover mode of operation. In either of these modesof operation, a BOP 60 is connected to the top of the tubing spool 14and a THRT 62 is attached to the top of the tubing hanger 18. The BOPincludes an internal BOP bore 64, at least one set of rams 66 which iscapable of sealing against the THRT 62, and at least one choke and killline 68 for providing communication between the BOP bore below the rams66 and a surface vessel (not shown). In addition, the THRT 62 comprisesan internal bore 70, or production port, which connects to theproduction bore 26 via a production stab (not shown). Also, although theBOP rams are described herein as sealing against the THRT, it should beunderstood that the rams could instead seal against another member, suchas an extension member, which comprises a production port thatcommunicates with the production port of the THRT.

[0042] During both installation and workover of the flow completionassembly 10, communication between the tubing annulus 24 and the surfacevessel may be established through the annulus passageway 32, theworkover passageway 36, the central bore 16, the BOP bore 64, and thechoke and kill line 68. For example, deep well circulation can beaccomplished by pumping fluid down the THRT bore 70, through theproduction bore 26, through the tubing string 22, up the tubing annulus24, through the annulus passageway 32, through the workover passageway36, into the central bore 16 above the tubing hanger 18, into the BOPbore 64 and through the BOP choke and kill line 68.

[0043] However, the flow through the central bore 16 above the tubinghanger 18 is relatively unrestricted, and this flow can foul the tubinghanger lockdown mechanism and erode the central bore, including thelockdown profile and the annular sealing surface within the central boreagainst which the tree cap 20 must lock and seal, respectively. If thetubing hanger lockdown mechanism becomes fouled, recovery of the tubinghanger from the tubing spool may be complicated. Additionally, if thetree cap cannot lock securely to the tubing spool 14 and form aneffective seal against the central bore 16, then the flow completionassembly 10 will not provide the required second barrier between thewell bore and the environment.

[0044] The present invention addresses the above-described limitationsin prior art flow completion assemblies by providing for communicationbetween the workover passageway and the BOP bore through the THRT and bymounting both the first and the second barriers solely on the tubinghanger.

[0045] Referring to FIG. 3, one embodiment of a flow completionapparatus according to the present invention, which is indicatedgenerally by reference number 110, is similar in many respects to theflow completion assembly 10 described above. Accordingly, the flowcompletion apparatus comprises a wellhead 112, a tubing spool 114 whichis mounted on the wellhead and which includes a central bore 116extending axially therethrough, and a generally annular tubing hanger118 which is supported on a shoulder (not shown) located in the centralbore and from which is suspended a tubing string 120 that extends intothe well bore and defines a tubing annulus 122 surrounding the tubingstring. As in the prior art flow completion assembly 10, the tubinghanger 118 is secured to the tubing spool 114 by a lockdown mechanism(not shown) and includes a production bore 124 which communicates withthe interior of the tubing string 120 and a lateral productionpassageway 126 which extends between the production bore and the outerdiameter of the tubing hanger. Similarly, the tubing spool 114 includesa production outlet 128 which communicates with the productionpassageway 126, an annulus passageway 130 which communicates with thetubing annulus 122, and an annulus outlet 132 which is connected to theannulus passageway. In addition, the tubing hanger 118 is sealed to thetubing spool 114 by an upper, preferably metal production seal ring 134and a lower, preferably metal production seal ring 136, each of whichengages a corresponding annular sealing surface formed on the centralbore 116. Furthermore, the production bore 124 is sealed above theproduction passageway 126 by a suitable closure member 138, such as awireline crown plug, which directs the flow of oil or gas from thetubing string 120 into the production passageway 126.

[0046] In accordance with the present invention, the tubing hanger 118also includes an annulus bore 140 which extends between the top and theouter diameter of the tubing hanger, and the tubing spool 114 comprisesa workover passageway 142 which extends between the annulus passageway130 and the annulus bore 140. In this manner, communication between thetubing annulus 122 and the top of the tubing hanger 118 is provided bythe annulus passageway 130, the workover passageway 142, and the annulusbore 140. As will be described below, this arrangement will permitcommunication between the tubing annulus 122 and a BOP to be routedthrough a THRT, rather than into the portion of the central bore 116above the tubing hanger 118.

[0047] Similar to the flow completion assembly 10, the flow completionapparatus 110 may also comprise a production master valve 144 and aproduction wing valve 146 to control flow through the production outlet128, and an annulus master valve 148, an annulus wing valve 150 and aworkover valve 152 to control flow through the annulus passageway 130,the annulus outlet 132 and the workover passageway 142, respectively.While these valves may be any suitable closure members, they arepreferably remotely operable gate valves. Moreover, some or all of thevalves may be incorporated into the body of the tubing spool 114, intoseparate valve blocks which are bolted onto the tubing spool, or intoindividual valve assemblies which are connected to their respectiveoutlets or passageways in the tubing spool with separate lengths ofconduit. Furthermore, the production outlet 128 and the annulus outlet132 are preferably connected to respective flow loops which communicatewith a surface vessel, either directly or via a manifold, in a mannerthat is well known in the art.

[0048] In the production mode of operation of the flow completionapparatus 110, which is depicted in FIG. 3, a first barrier between thewell bore and the environment is provided by the upper production seal134 and the closure member 138, which together serve to isolate thefluid in the production bore from the environment above the tubinghanger. In accordance with the present invention, a second barrierbetween the well bore and the environment is provided by a suitablesecondary closure member 154, such as a wireline crown plug, which ismounted in the production bore 124 above the closure member 138, and asecondary, preferably metal ring seal 156, which is mounted on thetubing hanger and sealingly engages a corresponding annular sealingsurface formed in the central bore 116. Together, the secondary closuremember 154 and the secondary seal ring 156 function to isolate the fluidin the production bore from the environment above the tubing hanger.Thus, the necessary first and second barriers for isolating theproduction bore from the environment are provided by components whichare mounted on or in the tubing hanger.

[0049] The present invention also provides for isolating the tubingannulus 122 from the environment above the tubing hanger 118 during theproduction mode of operation. Provided the annulus master valve 148 andthe workover valve 152 are closed, the upper production seal 134 and thesecondary seal 156 will provide the required first and second barriersbetween the tubing annulus and the environment. However, when pressurein the tubing annulus 122 needs to be bled off through the annuluspassageway 130 and the annulus outlet 132, or when gas is introducedinto the tubing annulus through the annulus outlet and the annuluspassageway during gas lift applications, the annulus master valve 148must be opened.

[0050] Therefore, the flow completion apparatus preferably alsocomprises a tree cap 158 which includes an annulus stab 160 that sealsinto the top of the annulus bore 140 to provide a second barrier, inconjunction with the workover valve 152, between the tubing annulus 122and the environment when the annulus master valve 148 is open. While thetree cap 158 may include an annular, preferably non-metallic seal (notshown) to seal against the tubing spool 114 and thereby prevent seawater from entering the central bore 116, the tree cap is not intendedto provide a barrier against well pressure in the production bore.Consequently, the tree cap 158 is a relatively lightweight member whichcan be installed using an ROV, thus eliminating the need to install thetree cap from a surface vessel. The tree cap 158 is preferably landed onthe tubing hanger 118 and locked to the tubing spool 114 with aconventional lockdown mechanism 162. This lockdown mechanism willprovide a backup to the lockdown mechanism used to secure the tubinghanger to the tubing spool. It should be noted that, although the treecap 158 is depicted as an internal tree cap, it could instead beconfigured as an external tree cap. Also, although not shown in thedrawings, the tree cap 158 could be locked directly to the tubing hanger118 rather than the tubing spool 114.

[0051] Referring now to FIG. 4, during installation and workover of theflow completion apparatus 110, a BOP 164 is lowered on a riser (notshown) and connected and sealed to the top of the tubing spool 114. TheBOP 164 includes an internal BOP bore 166, at least one choke and killline 168 and, in the embodiment of the invention shown in FIG. 4,preferably two sets of BOP rams or bags 170 and 172. In addition, a THRT174 is connected to the top of the tubing hanger 118. The THRT is eitherconnected to the tubing hanger at the surface vessel and used to lowerthe tubing hanger into the tubing spool during installation of thetubing hanger, or lowered through the riser and the BOP and connected tothe tubing hanger in the tubing spool in anticipation of a workoveroperation. The THRT 174 is shown to comprise a generally cylindricalouter diameter surface, a production port 176 which is connected to theproduction bore 124 by a suitable production seal stab 178, and anannulus port 180 which extends from a portion of the outer diametersurface to a suitable annulus seal stab 182 that engages the tubinghanger annulus bore 140.

[0052] Thus, with the BOP rams 170, 172 sealed against the THRT 174,communication between the tubing annulus 122 and the BOP choke and killline 168 may be established through the annulus passageway 130, theworkover passageway 142, the annulus bore 140, the annulus port 180, andthe portion of the BOP bore 166 which is located between the BOP rams170, 172. For example, with the annulus wing valve 150 closed, pressurecan be transmitted from the surface vessel down the choke and kill line168, through the annulus port 180, through the tubing hanger annulusbore 140, through the workover passageway 142, through the annuluspassageway 130, and into the tubing annulus 122 to test the integrity ofthe down hole packer (not shown). Also, deep well circulation can beaccomplished by closing both the annulus wing valve 150 and theproduction master valve 144 and pumping fluid down the choke and killline 168, through the annulus port 180, through the annulus bore 140,through the workover passageway 142, through the annulus passageway 130,down the tubing annulus 122, past the down hole packer, up the tubingstring 120, through the production bore 124, and up the production port176. Moreover, since the flow between the tubing hanger annulus bore 140and the choke and kill line 168 is restricted by the THRT, nopossibility exists that the flow will foul the tubing hanger lockdownmechanism or erode the central bore 116.

[0053] Referring now to FIG. 5, the flow completion apparatus 110 isshown connected to a BOP 164 a, which is different from the BOP 164described above in that the choke and kill line 168 of BOP 164 a entersthe BOP bore 166 below a single or the lower set of BOP rams 172. Inthis embodiment, fluid communication between the tubing annulus 122 andthe choke and kill line 168 is established by the annulus passageway130, the workover passageway 142, the annulus bore 140, the annulus port180, and the portion of the BOP bore 166 which is located below the BOPrams 172. While this arrangement allows the fluid to contact the centralbore 116 above the tubing hanger 118, once the annular area between theTHRT 174 and the central bore is filled with fluid, this area becomes ineffect a “dead zone” through which the fluid will not flow with anyintensity. Therefore, the flow of fluid will essentially be restrictedto the portion of the BOP bore 166 which is proximate the area whereboth the choke and kill line 168 and the annulus port 180 enter the BOPbore. Consequently, this fluid flow arrangement will not subject thecentral bore 116 to undue erosion.

[0054] Referring to FIG. 6, the flow completion apparatus 110 is shownin conjunction with a light well intervention package 184, which istypically used to monitor the tubing annulus 122 and service the wellfrom a workover vessel. After the tree cap 158 is removed, theintervention package 184 is landed and sealed to the top of the tubingspool 114. The intervention package 184 includes a production bore 186,which is connected to the production bore 124 by a suitable productionseal stab 188, and an annulus port 190, which is connected to the tubinghanger annulus bore 140 by a suitable annulus seal stab 192. Theintervention package also comprises a production closure member 194 forcontrolling flow through the production bore 186 and an annulus closuremember 196 for controlling flow through the annulus port 190. While theclosure members 194, 196 may be any suitable valves, they are preferablyremotely operable gate valves. In addition, at least the productionclosure member 194 is preferably a wire shearing gate valve.

[0055] As shown in FIG. 6, fluid communication between the tubingannulus 122 and the annulus port 190 in the intervention package 184 isestablished by the tubing hanger annulus bore 140, the workoverpassageway 142 and the annulus passageway 130. Consequently, the centralbore 116 above the tubing hanger 118 is isolated from the fluid flow.Therefore, the central bore will not be subject to erosion while serviceoperations are being performed with the intervention package 184.

[0056] Another embodiment of a flow completion apparatus according tothe present invention is shown in FIG. 7. The flow completion apparatusof this embodiment, which is indicated generally by reference number210, is shown to comprise all of the elements of the flow completionapparatus 110 discussed above. However, the tubing spool 114 a of theflow completion apparatus 210 also includes a crossover flow loop 212extending between the annulus outlet 132 and a portion of the productionoutlet 128 that is preferably located between the production mastervalve 144 and the production wing valve 146. In addition, the flowcompletion apparatus 210 comprises a suitable crossover valve 214, suchas a remotely operable gate valve, to control the flow through thecrossover flow loop 212.

[0057] With this arrangement, the flow completion apparatus 210 iscapable of providing communication between a surface vessel and eitherthe tubing annulus 122 or the production bore 124. With the annulus wingvalve 150 closed, an annulus communication path can be establishedthrough the BOP choke and kill line 168, the annulus port 180, theannulus bore 140, the workover passageway 142, and the annuluspassageway 130. Furthermore, with the annulus master valve 148 and theproduction wing valve 146 both closed, a production bore communicationpath can be established through the BOP choke and kill line 168, theTHRT annulus port 180, the tubing hanger annulus bore 140, the workoverpassageway 142, the annulus outlet 132, the crossover flow loop 212, theproduction outlet 128, and the production passageway 126. Provided adown hole safety valve or similar closure member (not shown) is closedto seal off the production string 120, the production bore communicationpath can be used to circulate fluid up the production port 176 for riserwashout operations. Provided further that the production port 176 isplugged, the production bore communication path can be used to pressuretest the tubing hanger seals 134 and 136. Persons of ordinary skill inthe art will recognize that other communication paths and otheroperations may also be achieved with the flow completion apparatus 210.

[0058] Referring now to FIG. 8, another embodiment of a flow completionapparatus, which is indicated generally by reference number 310, isshown to comprise many of the same components as the flow completionapparatus 110 described above. In this embodiment, however, the tree cap158 a does not include an annulus seal stab 160 for sealing the top ofthe annulus bore 140. Rather, the tubing hanger 118 a includes a stingopen-type valve 312, such as a poppet valve, mounted in the upper end ofthe annulus bore 140 to close off communication with the tubing annulus122 in the absence of an activating force applied from above, forexample, by a THRT. During workover operations with the flow completionapparatus 310, which is illustrated in FIG. 9, a THRT 174 a is loweredthrough the BOP 164 a and landed on the tubing hanger 118 a. Uponlanding on the tubing hanger, the THRT will press down on the stem ofthe valve 312 and thereby open the valve. Fluid communication may thusbe established between the annulus bore 140 and the annulus port 180 inthe THRT.

[0059] In a variation of the flow completion apparatus 310 which is notillustrated in any particular Figure, the male member of a conventionalfluid coupling assembly could be mounted at the top of the tubing hangerannulus bore and the corresponding female member of the assembly mountedin the bottom of the THRT annulus port. Thus, when the THRT 174 islanded on the tubing hanger, the male and female coupling members willengage and a sealed fluid path will be established between the tubinghanger annulus bore and the THRT annulus port.

[0060] Another embodiment of a flow completion apparatus according tothe present invention is shown in FIG. 10. The flow completion apparatusof this embodiment, which is indicated generally by reference number410, is similar to the flow completion apparatus 110 described above.However, the tubing hanger 118 b of this embodiment includes a crossoverbore 412 which extends between the annulus bore 140 and a portion of theproduction bore 124 located between the two closure members 138, 154. Inconjunction with a THRT such as 174 (not shown), and with the upper plug154 removed and the workover valve 152 closed, the crossover bore 412allows for circulation down the BOP choke and kill line (not shown),through the THRT annulus port (not shown), through the tubing hangerannulus bore 140, through the crossover bore 412, up the production bore124, and up the THRT production port (not shown). Other modes ofcirculation utilizing the crossover bore 412 may also be recognized bythose skilled in the art. Moreover, during production a first barrierbetween the production bore and the environment is provided by the upperproduction seal 134 and the lower closure member 138, while a secondbarrier is provided by the upper production seal 156, the upper closuremember 154, and the annulus seal stab 160 of the tree cap 158.Therefore, in addition to the advantages of the previous flow completionapparatuses discussed above, the flow completion apparatus 410 alsofacilitates certain circulation modes without the need for an externalcrossover flow loop.

[0061]FIG. 11 illustrates yet another embodiment of a flow completionapparatus in accordance with the present invention. The flow completionapparatus of this embodiment, generally 510, differs from the flowcompletion apparatus 110 described above in that the tubing hanger 118 cincludes a generally annular receptacle 512 formed at the top of theproduction bore 124, and the THRT 174 b comprises a preferably integralstab 514 which depends downwardly from the bottom of the THRT around theproduction port 176. When the THRT is landed on the tubing hanger 118 c,the stab 514 is received in the receptacle 512 to connect the productionbore 124 to the production port 176. The THRT 174 b ideally alsocomprises an annular, preferably non-metallic seal 516, for examplesurrounding the stab 514, for sealing the bottom of the THRT to the topof the tubing hanger 118 c around the production bore 124 and theproduction port 176. In the embodiment illustrated in FIG. 11, fluidcommunication between the tubing hanger annulus bore 140 and the THRTannulus port 180 is established by an annulus seal stab 182 carried onthe THRT. In this embodiment, fluid communication between the BOP chokeand kill line 168 and the tubing annulus 122 is established as describedabove in reference to FIG. 5.

[0062] Referring to FIGS. 12 and 13, another embodiment of a flowcompletion apparatus, generally 610, is shown to comprise many of thesame features as the flow completion apparatus 510. However, the flowcompletion apparatus 610 does not comprise an annulus seal stab 182between the THRT annulus port 180 and the tubing hanger annulus bore140. Rather, the THRT 174 c of this embodiment includes an annular lowerbody portion 612 which is located above the stab 514, and the tubinghanger 118 d comprises an annular rim 614 which extends generallyupwardly from the top of the tubing hanger. The rim 614 is adapted toreceive the lower body portion 612 when the THRT is landed on the tubinghanger. Furthermore, the THRT 174 c and the tubing hanger 118 d aredesigned such that, when the THRT is fully engaged in the tubing hanger,an annular space or gallery 616 will be formed within the rim 614between the bottom of the THRT and the top of the tubing hanger. In thismanner, fluid communication between preferably multiple THRT annulusports 180 and the tubing hanger annulus bore 140 is established throughthe gallery 616. The THRT 174 c may also include an annular seal 618around the lower body portion 612 to seal against the rim 614 andthereby more effectively isolate the annulus fluid flow. With theexception that the fluid flows between the THRT annulus ports 180 andthe tubing hanger annulus bore 140 through the gallery 616, fluidcommunication between the BOP choke and kill line 168 and the tubingannulus 122 is as described above in reference to FIG. 5.

[0063] A preferred embodiment of the flow completion apparatus is shownin FIGS. 14-17. In this embodiment of the invention, the flow completionapparatus, which is indicated generally by reference number 710, isshown to comprise a tubing spool 712 which is secured and sealed to awellhead housing 714 and which includes a central bore 716 extendingaxially therethrough, a tubing hanger 718 which is landed on a shoulder720 that is located in the central bore, and a THRT 722 which is securedto the top of the tubing hanger during installation and workoveroperations. The tubing hanger 718 supports a tubing string 724 whichextends into the well bore and defines a tubing annulus 726 surroundingthe tubing string. As in the previous embodiments, the tubing hanger 718includes a concentric production bore 728 which extends completelytherethrough and communicates with the interior of the tubing string724, a lateral production passageway 730 which extends between theproduction bore and the outer diameter of the tubing hanger, and anannulus bore 732 which extends between the outer diameter of the tubinghanger and the top of the tubing hanger. Similarly, the tubing spool 712includes a production outlet 734 which communicates with the productionpassageway 730, an annulus passageway 736 which extends between thetubing annulus 726 and an annulus outlet 738, and a workover passageway740 which communicates between the annulus outlet and the annulus bore732. The production outlet 734 and the annulus outlet 738 are preferablyconnected to respective flow loops which communicate with a surfacevessel, either directly or via a manifold, in a manner that is wellknown in the art.

[0064] The flow completion apparatus 710 also comprises a productionmaster valve 742, an annulus master valve 744 and a workover valve 746for controlling flow through the production outlet 734, the annuluspassageway 736 and the workover passageway 740, respectively. Thesevalve may be any suitable closure members, but are preferably remotelyoperable gate valves. Although not shown in the drawings, but similar tothe previous embodiments described above, the tubing spool 712preferably also includes a production wing valve located outboard of theproduction master valve 742 and an annulus wing valve positioned in theannulus outlet 738. Furthermore, the tubing spool 712 may also comprisea crossover flow loop and a crossover valve similar to that describedwith reference to FIG. 7. Moreover, some or all of the above-mentionedvalves may be incorporated into the body of the tubing spool 712, intoseparate valve blocks which are bolted onto the tubing spool, or intoindividual valve assemblies which are connected to their respectiveoutlets or passageways in the tubing spool with separate lengths ofconduit.

[0065] During the production mode of operation of the flow completionassembly 710, which is not shown in the drawings, the production bore728 is sealed above the production passageway 730 by suitable upper andlower closure members 748 and 750, which are shown in phantom in FIG.14. The closure members 748, 750 are preferably wireline crown plugswhich are received in corresponding profiles that are formed in a boreinsert 752 that is secured and sealed in the upper portion of theproduction bore 728. In addition, a test port (not shown) is ideallyrouted between the two plugs in a conventional fashion to prevent thecreation of a hydraulic lock during installation of the upper plug andto facilitate the testing of both plugs after they are installed. Afirst pressure-containing barrier between the well bore and theenvironment above the tubing hanger 718 is provided by the lower closuremember 750 and an annular upper production seal 754 which is positionedbetween the tubing hanger and the central bore 716 above the productionpassageway 730. A second pressure-containing barrier between the wellbore and the environment is provided by the upper closure member 748 andan annular secondary seal 756 which is positioned between the tubinghanger and the central bore above the upper production seal 754. Thus,both of the required first and second pressure-containing barriers aremounted in or on the tubing hanger in the preferred embodiment of theinvention.

[0066] As shown more clearly in FIG. 15, the upper production seal 754is ideally part of a seal assembly 758 that also includes an annularlower production seal 760 which is positioned between the tubing hanger718 and the central bore 716 below the production passageway 730. Eachproduction seal 754, 760 preferably comprises a straight bore-type metalseal with an inner radial sealing lip that engages the tubing hanger andan outer radial sealing lip that engages a corresponding annular sealingsurface 762 which is formed on the central bore 716. In addition, theproduction seals 754, 760 are optimally oriented so that the sealinglips will be energized into engagement with their corresponding sealingsurfaces by the pressure in the production passageway 730. Theproduction seals are spaced apart on the tubing hanger 718 by a spacerring 764, which includes a lateral hole 766 that aligns with theproduction passageway 730, and the entire seal assembly 758 is securedto the tubing hanger by a retainer ring 768. Furthermore, in theembodiment of the invention shown in FIG. 15, the seal assembly 758includes an upper backup seal ring 770, which is mounted in acorresponding groove formed in the tubing hanger above the upperproduction seal 754, and a lower backup seal ring 772, which is mountedin a corresponding groove formed in the body of the lower productionseal 760. While, the backup seal rings 770, 772 may be any suitableseals, they are preferably non-metallic face-type seals.

[0067] Referring still to FIG. 15, the secondary seal 756 preferablycomprises a straight bore-type metal seal with an inner radial sealinglip that engages the tubing hanger and an outer radial sealing lip thatengages a corresponding annular sealing surface 774 which is formed onthe central bore 716. Moreover, the secondary seal is ideally orientedso that the sealing lips will be energized by the pressure in theproduction passageway 730 should the upper production seal 754 fail. AT-ring 776 is provided to secure the secondary seal 756 against alateral step 778 that is formed in the outer diameter of the tubinghanger 718. The T-ring includes a stem which extends between the sealinglips and engages a base 780 of the secondary seal to force the secondaryseal firmly against the step as the T-ring is threaded onto the tubinghanger. Furthermore, flow completion apparatus 710 may comprise asecondary backup seal ring 782 that is mounted in a corresponding grooveformed in the tubing hanger above the secondary seal 756. The secondarybackup seal ring may be, for example, a non-metallic face-type seal.

[0068] Referring to FIG. 16, the tubing hanger 718 is provided with acylindrical upper extension or rim 784 that is threaded onto the upperend of the tubing hanger, a locking mandrel 786 which is telescopicallyreceived over the rim, and an expandable lock ring 788 which is carriedon an annular shoulder 790 that is formed on the outer diameter of thetubing hanger. The lower end of the locking mandrel 786 comprises awedge-shaped nose 792 which, when the mandrel is forced downward by theTHRT 722, is adapted to urge the lock ring 788 outward into acorresponding locking profile 794 formed in the central bore 716 tothereby lock the tubing hanger to the tubing spool 712. An anti-backoffring 796 which is secured to the upper end of the rim 784 includes aserrated outer surface that engages a corresponding grooved surface onthe inner diameter of the locking mandrel 786 to maintain the lockingmandrel in the locked position.

[0069] As seen most clearly in FIG. 17, the THRT 722 comprises a tubularmember 798 having an upper end which is connected to a string of drillpipe or the like (not shown), a cylindrical body 800 which is mounted onthe tubular member 798 and is secured thereto by suitable means, such asthreads 802, an annular retention sleeve 804 which is secured to theupper end of the body 800, a first cylindrical locking piston 806 whichincludes a first sleeve portion 808 that is slidably received over thebody 800 below the retention sleeve 804, and a second cylindricallocking piston 810 which includes a second sleeve portion 812 that isslidably received over the body 800 below the first sleeve portion 808.A collet finger ring 814 is attached to the lower end of the firstlocking piston 806, and a wedge ring 816 is attached to the lower end ofthe second locking piston 810.

[0070] In operation, the THRT 722 is lowered onto the tubing hanger 718until the bottom of the first locking piston 806 engages the top of thelocking mandrel 786 and the collet fingers 814 spring into acorresponding groove 818 formed on the inner diameter of the lockingmandrel. The second locking piston 810 is then stroked downward to trapthe collet fingers 814 in the groove 818 and thereby lock the firstlocking piston 806 to the locking mandrel 786. Simultaneously, the wedgering 816 will force an expandable locking ring 820 into a correspondinggroove 822 formed on the inner diameter of the rim 784 to thereby lockthe THRT 722 to the tubing hanger 718. Once the tubing hanger is landedin the tubing spool 712, the first locking piston 806 is strokeddownward to force the locking mandrel 786 downward, and this will forcethe lock ring 788 into the locking profile 794 to thereby lock thetubing hanger to the tubing spool, as was described above in connectionwith FIG. 16. The second locking piston 810 is then stroked upward tounlock the THRT 722 from the tubing hanger. This will also unlock thecollet fingers 814 from the locking mandrel 786 and therefore allow theTHRT to be retrieved while the locking mandrel remains in the lockedposition. The tubing hanger 718 can also be retrieved using the THRT 722by connecting the THRT to the tubing hanger as described above and thenstroking the first locking piston 806 upward to pull the locking mandrel786 upward and thereby allow the lock ring 788 to retract out of thelocking profile 794.

[0071] As shown in FIGS. 16 and 17, the THRT 722 comprises a productionport 824 within the tubular member 798 which communicates with a riser(not shown) in a manner well understood by those of skill in the art. Inaddition, the bottom of the tubular member forms a production seal stab826 which, when the THRT is connected to the tubing hanger 718, isreceived in the top of the production bore 728 to connect the productionport 824 with the production bore.

[0072] Furthermore, when the THRT 722 is connected to the tubing hanger718, the bottom of the body 800 is spaced apart from the top of thetubing hanger to thereby form an annular gallery 828 which is incommunication with the tubing hanger annulus bore 732. The gallery 828is sealed from the environment by a lower annular isolation seal 830,which is engaged between the tubing hanger 718 and the rim 784, and anupper annular isolation seal 832, which is engaged between the rim andthe outer diameter of the wedge ring 816. The seals 830, 832 may be anysuitable seals, but are preferably non-metallic face seals. The outerdiameter of the tubular member 798 is designed to be smaller than theinner diameter of the body 800 in order to form an annular volume or“annulus port” 834 between these two members which extends between thegallery 828 and the top of the body. The spacing between the tubularmember and the body is ideally maintained by a number of flutedcentralizers 836 which may be attached to or formed integrally witheither the tubular member or the body. Moreover, the threads 802 whichsecure the body 800 to the tubular member 798 are fluted to allow forfluid to pass through this connection.

[0073] In this manner, when a BOP (not shown) is connected to the tubingspool 712 and the BOP rams are closed around the tubular member 798,fluid communication between the BOP choke and kill line and the tubinghanger annulus bore 732 is established through the annulus port 834 andthe gallery 828. If the BOP choke and kill line enters the BOP borebelow the BOP rams, then fluid communication between the BOP choke andkill line and the tubing annulus 726 may be established in a mannersimilar to those described above in connection with FIGS. 5 and 12, andvarious fluid circulation paths may be formed through the flowcompletion apparatus 710 as described, for example, in connection withFIGS. 5 and 7. If, however, the BOP choke and kill line enters the BOPbore between two sets of BOP rams, then fluid communication between theBOP choke and kill line and the tubing annulus 726 may be established ina manner similar to that described above in connection with FIG. 4.

[0074] Referring again to FIG. 16, the flow completion apparatus 710preferably also includes a debris valve 838 to allow fluid to passthrough the annulus bore 732 but prevent debris from falling into theannulus bore when the THRT 722 is removed from the tubing hanger 718.The construction and operation of the debris valve are explained morefully in U.S. Pat. No. 6,497,277, which is commonly owned herewith andis hereby incorporated herein by reference.

[0075] While in the production mode, the flow completion apparatus 710may comprise a tree cap similar to the tree cap 158 described above.Accordingly, as shown most clearly in FIG. 17, the central bore 716preferably includes a locking profile 840 against which the tree caplockdown mechanism may engage, and the top of the annulus bore ideallyincludes a seal profile 842 against which the tree cap annulus stab mayseal.

[0076] Referring again to FIG. 14, the flow completion apparatuspreferably comprises a number of service and control conduits which arearranged radially about the central axis of the tubing hanger. Theservice and control conduits are bores which are formed in the body ofthe tubing hanger 718 from either the top or the outer diameter of thetubing hanger and which extend into or completely through the tubinghanger. The service and control conduits primarily function to provideaccess into or through the tubing hanger from positions above orradially outside the tubing hanger. For example, the service and controlconduits may comprise a number of fluid transfer ports for communicatinghydraulic control fluid to valves or other known devices located in orbelow the tubing hanger. In addition, the service and control conduitsmay include one or more fluid transfer ports for conveying chemicals orother well service fluids through the tubing hanger to select downholelocations. Also, the service and control conduits may comprise a numberof signal transfer ports which accommodate electrical, optical orsimilar cables for conveying power to corresponding devices positionedin or below the tubing hanger.

[0077] In the embodiment of the invention depicted in FIG. 14, forexample, a service and control conduit 844 a for providing hydrauliccontrol fluid to a surface controlled subsea safety valve (“SCSSV”) (notshown) which is positioned in the tubing string 724 is coupled to acorresponding conduit 846 a located in the THRT 722. The conduit 846 ain turn is connected to a control panel on the surface vessel so thatthe status of the SCSSV may be monitored during installation of thetubing hanger 718. The conduits 844 a, 846 a may be releasably coupledusing a conventional poppet-type fluid coupling. However, as shown moreclearly in FIG. 16, the conduit 844 a may alternatively be connected toa male coupling 848 which comprises a combination poppet and gate valve,such as is described more fully in U.S. Pat. No. 6,520,207, which iscommonly owned herewith and is hereby incorporated herein by reference.In addition the conduit 846 a is connected to a female coupling 850which forms a part of a standard hydraulic coupling. In this manner, thecoupling 848 will provide two barriers between the conduit 844 a and theenvironment.

[0078] In a similar fashion, a service and control conduit in the tubinghanger (not shown) may provide an electrical conduit to a pressuresensor (not shown) located in the tubing annulus 726. This conduit wouldbe connected to a corresponding conduit 846 b in the THRT 722, which inturn would be connected to the control panel located on the surfacevessel so that the pressure in the tubing annulus can be monitoredduring installation of the tubing hanger 718. These conduits arepreferably releasably connected using a conventional electrical coupler,the female portion 852 of which is shown in FIG. 16 connected to theconduit 846 b.

[0079] The flow completion apparatus 710 may also include one or moremale radial penetrator couplings 854, each of which is connected to acorresponding service and control line 844 c within the tubing hanger718. The male couplings 854 are mounted on the outer diameter of thetubing hanger in a conventional fashion, and each male coupling isreleasably connectable with a corresponding female coupling (not shown)that is carried by a conventional radial penetrator assembly which ismounted on the tubing spool 712. Thus, when the tubing hanger is landedon the tubing spool, the penetrator assembly is actuated to bring thefemale couplings into engagement with their corresponding male couplings854 to thereby establish communication between the service and controlconduits 844 c in the tubing hanger 718 and the external conduits towhich the female couplings are connected. It should be noted that, oneor more of the service and control conduits 844 c may be connectedwithin the tubing hanger to a corresponding service and control conduitwhich enters from the top of the tubing hanger. In this manner, a devicewith which the THRT 722 communicates during installation of the tubinghanger, for example an SCSSV, can be connected to an external serviceand control line through the tubing spool 712 once the THRT isdisconnected from the tubing hanger.

[0080] In order to ensure that the tubing hanger 718 is properlyangularly oriented in the tubing spool 712, and that the male couplings854 are therefore properly aligned with the female couplings, the flowcompletion apparatus 710 preferably includes a fine alignment key 856,which ideally is bolted onto the outer diameter of the tubing hanger. Asthe tubing hanger is landed in the tubing spool, the fine alignment keywill engage a corresponding tapered slot in the central bore 716 andthereby force the tubing hanger to achieve the orientation defined bythe tapered slot.

[0081] Yet another embodiment of the present invention is shown in FIG.18. The flow completion apparatus of this embodiment, which is indicatedgenerally by reference number 910, is shown to be similar in manyrespects to the flow completion apparatus 710 discussed immediatelyabove. However, in this embodiment of the flow completion apparatus 910,the tubing hanger 718 comprises a single “dual-sealing” wireline plug912 to seal the production bore 728 above the production passageway 730.In addition, the tubing hanger 718 includes a radially oriented annulusgallery 914 which is formed on the outer diameter of the tubing hangerbetween the secondary seal 756 and an annular tertiary seal 916, such asa non-metallic face-type seal, that is positioned between the tubinghanger and the central bore 716. Furthermore, the tubing hanger annulusbore 732 comprises a number of branches which extend from the annulusgallery 914 to the top of the tubing hanger. Finally, the flowcompletion apparatus 910 preferably comprises a light-weight, nonpressure-containing tree cap 918 which is installed in the tubing spool712 above the tubing hanger 718, and an optional debris cap 920 which isinstalled on the tubing spool 712 above the tree cap 918.

[0082] The dual-sealing plug 912 comprises a wireline deployable plugbody 922 which is removably connectable within the bore insert 752 thatis secured and sealed in the production bore 728. In addition, thedual-sealing plug 912 includes a first annular sealing assembly 924 forsealing between the body 922 and the bore insert 752, and a secondannular sealing assembly 926 for sealing between the body and,preferably, the production bore 728. Moreover, the first and secondsealing assemblies 924, 926 each ideally include at least one metalstraight-bore type ring seal. In this manner, the single plug 912performs the function of the two individual plugs 748, 750. However,only one running trip is required to install or remove the plug 912, asopposed to separate trips to install or remove each of the plugs 748,750.

[0083] Referring to FIGS. 19 and 19A-19C, the tree cap 918 is shown tocomprise an annular body 928 which includes a number of radial sectionsthat are secured together by a plurality of longitudinal bolts 930. Thebody 928 is preferably manufactured from ultra-high molecular weighpolyethylene, which has a very low water adsorption rate on the order ofabout 0.03% in the 24 hour ASTM D570 test. This material not only makesthe tree cap 918 lightweight, thereby allowing the tree cap to beinstalled by an ROV, but also isolates the tree cap from the cathodicprotection system of the flow completion apparatus. Moreover, anylongitudinal forces acting on the tree cap will be borne by the bolts930, thereby relieving the body 928 of this function.

[0084] Referring specifically to FIG. 19B, the tree cap 918 alsocomprises a collet sleeve 932 which is threaded onto the outer diameterof the body 928 near the top of the tree cap. The collet sleeve 932includes a number of downwardly depending collet fingers 934 which areadapted to engage a corresponding groove that is formed in the upper rim784 of the tubing hanger 718 when the tree cap 918 is landed in thetubing spool 712. The tree cap further includes an elongated lockmandrel 936 which comprises a camming ring 938 that is attached to alower end thereof, a plurality of locking dogs 940 which are slidablyreceived in corresponding radial apertures that are formed in the body928, and a landing ring 942 which is secured by a number of lock downscrews 944 in a support sleeve 946 that is threaded onto the colletsleeve 932. Also, as shown in FIG. 19C, the tree cap 918 preferablyincludes a key 948 that comprises a radially extending tongue 950 whichis received in a corresponding recess that is formed on the lock mandrel936. The key 950 serves to maintain the lock mandrel 936 in the upposition until the tree cap 918 is landed in the tubing spool 712.

[0085] When the tree cap 918 is landed in the tubing spool 712, thelanding ring 942 will land on the top of the tubing hanger lockingmandrel 786 and the collet fingers 934 will enter their correspondinggroove on the upper rim 784 (see FIG. 18). After the ROV turns the key948 to release the lock mandrel 936, the lock mandrel is pushed downwardby an ROV handling tool (not shown) to force the camming ring 938against the locking dogs 940, which in turn will move radially outwardlyagainst the collet fingers 934 to secure the collet fingers in theirgroove. The longitudinal spacing between the landing ring 942 and thecollet fingers 934 can be adjusted using the lockdown screws 944. Also,once the tree cap 918 is secured to the tubing hanger 718, the lock downscrews 944 can be tightened by an ROV torque tool 952 to firmly securethe landing ring 942 against the tubing hanger locking mandrel 786. Inthis manner, the landing ring 942 will function to maintain the tubinghanger locking mandrel 786 in the locked position.

[0086] As shown in FIG. 19A, the tree cap 918 can include a conduit 954which comprises a locking profile for an electrical connector. Thus, thetree cap can facilitate connecting an external electrical service andcontrol line to a corresponding service and control conduit in thetubing hanger 718. The tree cap 918 may also include an ROV hot stab 956through which a corrosion inhibitor may be injected into the centralbore 716 surrounding the tree cap.

[0087] As shown in FIGS. 19B and 19C, the tree cap 918 may also comprisean annulus seal stab 958. The seal stab 958 ideally comprises a threadedstem 960 which is received in a corresponding threaded receptacle 962that is attached to the lower end of an actuating shaft 964. Inaddition, the seal stab 958 is optimally keyed to a surroundingreceptacle 966 in a manner which prevents rotation but permitslongitudinal movement of the seal stab relative to the receptacle. Thus,rotation of the actuating shaft by a suitable ROV tool (not shown) willmove the seal stab 958 downward into engagement with, for example, theannulus bore 732 in the tubing hanger 718. The seal stab 958 may be ablind stab, in which event it functions to provide a backup barrier tothe annulus bore 732. Alternatively, the seal stab 958 may comprise athrough bore 968 which communicates through a corresponding conduit 970with a fluid coupling 972 that is mounted in the top of the tree cap918. In this manner, the pressure in the tubing annulus 726 may bemonitored through the seal stab 958 and a corresponding external serviceand control line which is attached to the coupling 972, or the seal stabcan be used to convey gas or other fluids from the external service andcontrol line into the tubing annulus.

[0088] Referring to FIGS. 18 and 20, the debris cap 920 is preferably aseparate member which is mounted to the top of the tubing spool 712after the tree cap 918 is installed. The debris cap 920 comprises anannular body which includes an outer rim 974 that is sized to fit aroundthe outer diameter of the tubing spool 712 and an inner rim 976 that isadapted to fit closely around the upper end of the tree cap body 928.The inner rim 976 defines an enlarged opening 978 in the debris cap 920through which the top of the tree cap 918 may be accessed. In addition,the debris cap 920 preferably includes a first annular seal 980 which ispositioned between the inner rim 976 and the tree cap 918 and a secondannular seal 982 which is positioned between the outer rim 974 and thetubing spool 712. The seals 980, 982 function to keep sea water out ofand corrosion inhibitor in the central bore 716 around the tree cap 918.

[0089] The debris cap 920 is secured to the tubing spool 712 bypreferably two locking pins 984, each of which is rotatably received ina cylindrical housing 986 that is attached to the outer rim 974. Eachpin 984 includes a radially extending lug 988 which slidably engages acorresponding dogleg groove 990 that is formed in the housing 986. Inaddition, the debris cap 920 includes a handle 992 which is connected toboth pins 984. When the handle 992 is in the raised position, the lugs988 will occupy the radial outer portion of the dogleg groove 990 andthe pins will be in a radially outward position. When the handle 992 islowered, the lugs 988 will follow the dogleg groove 990 radiallyinwardly and thereby force the pins into engagement with an annulargroove which is formed on the outer diameter of the tubing spool 712 tothereby lock the debris cap to the tubing spool.

[0090] In a variation of the flow completion apparatus of the presentinvention, the seal which is associated with the annulus stab in theTHRT and the seal which is positioned between the tubing hanger and thetubing spool above the workover passageway, such as the seal 916 shownin FIG. 18, need not be pressure containing seals. Rather, they cansimply be elastomer or metal seals that are provided to prevent thepassage of debris and to direct the annulus fluid along the preferredflow path through the tubing hanger annulus bore and the THRT annulusport. Thus, while the central bore of the tubing spool will be exposedto pressure equal to the pressure in the tubing annulus, the debris inthe annulus fluid will be blocked by the above-mentioned seals and willtherefore not be allowed to foul the tubing hanger lockdown mechanism orerode the central bore.

[0091] In yet another variation of the present invention, the seal whichis associated with the annulus stab in the THRT and the seal which ispositioned between the tubing hanger and the tubing spool above theworkover passageway, such as the seal 916 shown in FIG. 18, may beeliminated entirely. Instead, the landing seat of the tubing hanger,that is, the seat which engages the landing shoulder in the central borefrom which the tubing hanger is suspended, is located above the workoverpassageway. The contact between the landing seat and the landingshoulder is sufficiently strong to prevent the flow of debris past thelanding seat. In addition, the annulus stab is a non-sealing metal stabwhich has a radial clearance with the annulus bore that is between about0.005 inch and 0.010 inch. This clearance is sufficiently small toprevent the significant passage of debris.

[0092] In a preferred embodiment of the invention, the flow completionapparatus ideally comprises means for isolating the central bore of thetubing spool from any fluid flowing between the BOP choke and kill lineand the THRT annulus port. In the embodiment of the invention shown inFIG. 4, for example, such isolation means is provided by the lower BOPrams 172. In an alternative embodiment of the invention which is shownin FIG. 21, the isolation means may comprise a seal 994 which is carriedon the THRT 174 and which seals against the BOP bore 166 below theannulus port 180. As shown in FIG. 22, for example, the isolation meanscomprises a conventional, preferably non-metallic radial seal 996 whichis mounted on the retention sleeve 804 of the THRT 722. In thisarrangement, fluid flowing between the choke and kill line 168 and theTHRT annulus port 834 will be confined to that portion of the BOP bore166 located between the seal 994 and the BOP rams 170. Thus, in each ofthese embodiments the fluid flowing between the BOP choke and kill lineand the THRT annulus port will be isolated from the central bore of thetubing spool, thereby eliminating the risk that the fluid will foul thetubing hanger lockdown mechanism or erode the central bore 116.

[0093] It should be recognized that, while the present invention hasbeen described in relation to the preferred embodiments thereof, thoseskilled in the art may develop a wide variation of structural andoperational details without departing from the principles of theinvention. For example, the various elements shown in the differentembodiments may be combined in a manner not illustrated above.Therefore, the appended claims are to be construed to cover allequivalents falling within the true scope and spirit of the invention.

What is claimed is:
 1. A flow completion apparatus for controlling theflow of fluid through a tubing string which extends through a wellheadhosing that is installed at an upper end of a well bore and defines atubing annulus surrounding the tubing string, the flow completionapparatus comprising: a tubing spool which is positioned over thewellhead housing and which includes a central bore that extends axiallytherethrough, a production outlet that communicates with the centralbore, and an annulus passageway that communicates with the tubingannulus; a tubing hanger which is supported in the central bore, isconnected to an upper end of the tubing string, and includes aproduction bore that extends axially therethrough and a productionpassageway that communicates between the production bore and theproduction outlet; a closure member which is positioned in theproduction bore above the production passageway; an annular seal whichis positioned between the tubing hanger and the central bore above theproduction passageway; wherein the tubing spool further comprises aworkover passageway which communicates the annulus passageway with aportion of the central bore that is located below the top of the tubinghanger; and wherein the tubing hanger further comprises an annulus borewhich extends between the workover passageway and the top of the tubinghanger; whereby fluid communication between the tubing annulus and thetop of the tubing hanger may be established through the annuluspassageway, the workover passageway and the annulus bore.
 2. The flowcompletion apparatus of claim 1, wherein the workover passageway extendsbetween the annulus passageway and a portion of the central bore that islocated above the annular seal.
 3. The flow completion apparatus ofclaim 1, further comprising: a BOP which is removably connectable to thetop of the tubing spool and which includes a BOP bore, a first set ofBOP rams, and at least one choke and kill line that communicates with aportion of the BOP bore which is located below the first BOP rams; and aTHRT which is removably connectable to the top of the tubing hanger andwhich includes an outer surface portion, a production port thatcommunicates with the production bore, and an annulus port thatcomprises a first end which communicates with the annulus bore and asecond end which communicates with the outer surface portion; whereinthe first BOP rams are adapted to sealingly engage the outer surfaceportion above the second end of the annulus port; whereby fluidcommunication between the tubing annulus and the BOP choke and kill linemay be established through the annulus passageway, the workoverpassageway, the annulus bore, the annulus port and the portion of theBOP bore which is located below the first BOP rams.
 4. The flowcompletion apparatus of claim 3, further comprising means for isolatingthe central bore from the second end of the annulus port.
 5. The flowcompletion apparatus of claim 4, wherein isolation means comprises asecond set of BOP rams which is adapted to sealingly engage the outersurface portion below the second end.
 6. The flow completion apparatusof claim 4, wherein the isolation means comprises an annular seal whichis mounted on the THRT below the second end and which sealing engagesthe BOP bore.
 7. A flow completion apparatus for controlling the flow offluid through a tubing string which extends through a wellhead housingthat is installed at an upper end of a well bore and defines a tubingannulus surrounding the tubing string, the flow completion apparatuscomprising: a tubing spool which is positioned over the wellhead housingand which includes a central bore that extends axially therethrough, aproduction outlet that communicates with the central bore, and anannulus passageway that communicates with the tubing annulus; a tubinghanger which is supported in the central bore, is connected to an upperend of the tubing string and includes a production bore that extendsaxially therethrough, a production passageway that communicates betweenthe production bore and the production outlet, a generally cylindricalouter diameter surface, and an upper surface; wherein the tubing hangerfurther comprises an annulus bore which extends between the outerdiameter surface and the upper surface; and wherein the tubing spoolfurther comprises a workover passageway which communicates between theannulus passageway and the annulus bore; whereby fluid communicationbetween the tubing annulus and the upper surface of the tubing hangermay be established through the annulus passageway, the workoverpassageway and the annulus bore.
 8. The flow completion apparatus ofclaim 7, further comprising: a BOP which is removably connectable to thetubing spool and which includes a BOP bore, a first set of BOP rams, andat least one choke and kill line that communicates with a portion of theBOP bore which is located below the first BOP rams; and a THRT which isremovably connectable to the tubing hanger and which includes acylindrical outer surface portion, a production port that communicateswith the production bore, and an annulus port that comprises a first endwhich communicates with the annulus bore and a second end whichcommunicates with the outer surface portion; wherein the first BOP ramsare adapted to sealingly engage the outer surface portion above thesecond end of the annulus port; whereby fluid communication between thetubing annulus and the BOP choke and kill line may be establishedthrough the annulus passageway, the workover passageway, the annulusbore, the annulus port and the portion of the BOP bore which is locatedbelow the first BOP rams.
 9. The flow completion apparatus of claim 8,further comprising means for isolating the central bore from the secondend of the annulus port.
 10. The flow completion apparatus of claim 9,wherein isolation means comprises a second set of BOP rams which isadapted to sealingly engage the outer surface portion below the secondend.
 11. The flow completion apparatus of claim 9, wherein the isolationmeans comprises an annular seal which is mounted on the THRT below thesecond end and which sealing engages the BOP bore.